Vacuum tube filament control



p 3, 1957 J. M. MCBEAN 2,805,385

VACUUM TUBE FILAMENT CONTROL Filed Jan. 7, 1946 INVENTOR JAMES M. McBEAN BY Ww ATTORNEY United States Patent Ofitice VACUUM TUBE FILAMENT CONTROL James M. McBean, Arlington, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application January 7, 1946, Serial No. 639,674

4 Claims. (Cl. 323-66) This invention relates to apparatus for controlling the alternating voltage supplied to a load, and more particularly to such apparatus for controlling the alternating current flowing in the filament of a vacuum tube, and especially the alternating current flowing in the filaments transmitting oscillators in which such control is often desirable.

Previous systems have used ballast devices and vacuum tube amplifier circuits for regulating filament voltage. The ballast device is usually an iron filament in series with the supply source whose characteristic is such that it provides a higher resistance if the supply voltage increases and vice-versa. This ballast device has proved to be unsatisfactory because its characteristic of providing increased resistance when current flow increases is not ideal, because its filament is quite susceptible to breakage when shocked, and because this system provides no method of controlling the amount of current at any value at all.

The vacuum tube amplifier devices for regulating filament current flow utilize either the load or line voltage to provide a constant voltage to the load irrespective of the line voltage fluctuations. This device is undesirable because it does not provide a means of boosting or lowering the filament current at will by means of trigger circuits or an associated circiut.

It is an object of this invention to provide an apparatus for electrically controlling or regulating the current delivered to a load.

Another object of this invention is to provide an electrical means of controlling the alternating current potential applied to a vacuum tube or tubes or other load at a time when either more or less current is needed.

A further object of this invention is to provide a method of controlling the amount of alternating current to an electrical load by using'the impedance change brought about by a saturable core reactor which is connected i'ri'series' with the load supply and whose impedance can becontrolled automaticallyby an associated circuit.

A still further object of this invention is to provide control over the alternating potential impressed upon the heaters of a vacuum tube so that the potential may be increased or decreased as desired, when the vacuum tube is expected to deliver more power.

Further objects and advantages of this invention, as well as its arrangement and operation will be apparent from the following description and claims in connection with the accompanying drawing which is a schematic diagram of an embodiment of this invention. -Referring-tothedrawing; it is seen that the line voltage is fed in at terminals and 11. 'This'alternating volt-' age is rectified by the bridge type rectifier 12 whose direct current output is fed to the bias coil 14 through the variable resistor 13. The setting of the variable resistor 13 determines the amount of current flowing in the bias coil 14 and in this application is adjusted so that 2,805,385 Patented Sept. 3, 1957 the core of the saturable core reactor 15 is saturated. The line voltage from terminals 10 and 11 is also fed to the primary 18 of the step down transformer 16 through the alternating current coils 19 and 20 indicated as being on the outer legs of the saturable core reactor 15 and which are connected in flux opposition. With the core of the saturable core reactor 15 saturated, the impedance of the windings 19 and 20 approaches zero and the whole of the line voltage appears across the primary 18 of the step down transformer 16. The voltage induced in the secondary winding 17 which is connected to terminals 21 and 22 is now relatively large and causes a corresponding relatively high alternating current to flow in the load which is also to be connected to terminals 21 and 22.

If a source of direct potential is connected to terminals 25 and 26, a current will flow in control winding 24, which preferably has equivalent turns to bias coil 14, on the saturable core reactor 15. The flux due to the current flow in the control winding 24 is in opposition to the flux due to the current flow in the bias winding 14. Thus the control winding is used to produce a flux to buck the flux produced by the bias winding 14 in the core of the saturable core reactor 15 which increases the impedance of windings 19 and 20 so that the portion of the line voltage appearing across the primary 18 of the transformer 16 has been reduced which likewise reduces the potential at terminals 21 and 22 thus reducing the current flowing in the load.

Thus when operating conditions require the reduction of the filament voltage, a direct current potential is applied to the control coil 24 whose fiux bucks out flux from the bias coil 14 to the degree required to increase the impedance of the windings 19 and 20 for the required load voltage. This direct current potential applied to terminals 25 and 26 may come from some part of the tube circuit (not shown on the drawing) or from an associated circuit.

The standard voltage regulator tube 23 which is connected through resistor 40 across terminals 25 and 26, and across the control coil 24 serves to limit the control flux to some maximum amount whose actual value is determined by the voltage rating of the voltage regulator tube.

It the voltage in the control coil 24 is equal to that in the bias coil 14 the magnetic iields neutralize each other. The impedance of the alternating current coils 19 and 20 will be a maximum and the load voltage a minimum. Should the direct current voltage fed to the control coil 24 be supplied by the plate circuit of a vacuum tube whose filaments are heated by connection to terminals 21 and 22, the minimum filament voltageismaintained, even though the plate current of the vacuum tube may increase, by the voltage regulator tube 23 which will fire. The ignition of the regulator tube 23, in parallel with the control coil 24, occurs when ever the voltage across that coil equals the voltage across the bias coil 14.

Since the ignition voltage of the voltage regulator tube 23 is somewhat higher than its operating voltage, the current in control coil 24 must increase until the potential required for ignition is reached before the tube acts as a regulator. This voltage difference is used to advantage as a certain time, due to the high inductance of the control coil 24, is required for the current to reach an equivalent value to that flowing through the bias coil 14. The control voltage then continues past the operating point up to the firing point of the voltage regulator tube, while the current lags behind by an interval of time approximately equal to the time required for the control voltage to increase to the firing point of the regulator tube 23. In other words, the flux in the control coil 24, which is in phase with the current, has reached the correct density at the approximate time the voltage has reached the firing point of the voltage regulator tube 23. The regulator tube then fires, the voltage across the control coil 24 immediately drops to the proper value, and the current value remains essen tially constant. The control voltage can then continue to increase with no further change. in current in the control coil 24.

It is to be understood that the circuit can also be used in a reverse manner to increase the load voltage at terminals 21 and 22 by removing or lowering the direct current potential applied to terminals 25 and 26.

it is also to be understood that the circuit can be used to control the load voltage by eliminating the bias coil and simply using the control voltage on the control coil 24 to increase or decrease the load voltage, or the bias coil 14 may be connected in an additive manner to the control coil 24 to accomplish control of the load voltage at terminals 21 and 22.

It is to be further understood that the control voltage may be applied to terminals 25 and 26 through some appropriate trigger circuits in the associated equipment.

This apparatus is easily adapted to regulating the load voltage at terminals 21 and 22 by connecting terminal 25 to terminal 27 and terminal 26 to terminal 28 so that irrespective of fluctuations of the alternating current supply voltage applied to terminals 10 and 11, the load voltage at terminals 21 and 22 will remain constant. The action of the saturable core reactor 15 is the same as described above. The direct current potential now applied to the control Winding 24 from the rectifier 12 is of sufficient value to ignite the voltage regulator tube 23 which will keep the current flowing in bias coil 14 constant for wide fluctuations of the line voltage supply. The flux produced by this winding is suflicient to saturate the core of the saturable core reactor 15. The current flowing in the bias winding 14 is again determined by the setting of the variable resistor 13. The variable resistor 13 is adjusted so that the flux produced by bias coil 14 bucks out enough of the control coil flux so that the core of the saturable core reactor 15 is magnetized along the linear portion of its magnetization curve below saturation. If the line supply voltage rises, the potential on the bias coil 14 increases which increases the flux produced in this coil and further reduces the total flux in the saturable core reactor 15. This increases the impedance of the alternating current windings 19 and 20 which reduces the output load voltage at terminals 21 and 22 as described above. The theory of operation for a decrease in line supply voltage is analogous.

It is to be understood that the regulation could be obtained equally well by connecting the rectifier 12 across the load terminals 21 and 22 thus regulating the load voltage irrespective of fluctuations of the current drawn by the load as well as fluctuations of the line sup ply voltage.

What is claimed is:

1. Apparatus for controlling the alternating voltage supplied to a load comprising, a rectifier having its input connected to the source of said alternating voltage, a variable resistor connected to the output of said rectifier, a saturable core reactor having a bias coil fed by the output of said rectifier through said variable resistor and a pair of alternating current coils in flux opposition, a transformer having its secondary adapted to be connected to said load, the primary of said transformer and said alternating current coils being connected in series across the input to said rectifier and in parallel with said rectifier, said saturable core reactor also having a control coil thereon and adapted to be connected to a source of direct voltage having a variable amplitude, a voltage regulator tube connected across said source of direct voltage to control the flux in said saturable core reactor in accordance with the magnitude of said direct voltage up to the rated voltage of said voltage regulator tube.

2. Apparatus for controlling the alternating voltage supplied to an alternating current load circuit comprising, a saturable core reactor having a biasing coil and a control coil thereon wound in flux opposition, and'a pair of alternating current windings wound in flux opposition, a source of alternating voltage, said alternating current windings and said load circuit being connected in series across said source of alternating voltage, a rectifier connected across said alternating voltage source in parallel with said windings and said load circuit, said biasing coil being connected across the output of said rectifier, means connected in series with said biasing coil for adjusting the flux in said reactor produced by said biasing coil, means connected to said control coil for controlling the current therein in proportion to the amplitude thereof a direct voltage present in said load circuit whereby the impedance of said reactor is correspondingly controlled, and means connected across said control coil for limiting the voltage applied to said control coil to a predetermined amount.

3. Voltage control means comprising, an alternating current supply, a load, a saturable core reactor, a pair of alternating current coils Wound on said saturable core reactor in flux opposition, said coils being in series circuit with said load, a rectifier connected across said load in parallel with said coil circuit, a biasing coil wound on said saturable core reactor and connected across the output of said rectifier, a control coil wound on said saturable core reactor in flux opposition to said biasing coil, a variable direct current source in circuit with said control coil, and a voltage regulator tube connected across said control coil for limiting the voltage applied thereto in a predetermined manner.

4. Control means forvacuum tube filaments comprising, an alternating current supply, a saturable core reactor having mounted thereon a plurality of coils, the first and second of said coils being wound in flux opposition and in series circuit with said supply and a load, the third and fourth coils also being wound in flux opposition, a rectifier connected across said supply in parallel with said coils and load circuit, the third coil being connected across the output of said rectifier, said fourth coil being connected across a variable direct current source, and a voltage regulator tube connected across the fourth coil for limiting the voltage applied thereto in a predetermined manner.

References Cited in the file of this patent UNITED STATES PATENTS 1,834,416 Osgood Dec. 1, 1931 1,986,112 Logan Jan. 1, 1-935 2,278,151 Runaldue Mar. 31', 1942 2,287,754 Barth June 23, 1942 2,331,411 Milarta Oct. 12, 1943 FOREIGN PATENTS 497,768 Great Britain Dec; 28, 1938 OTHER REFERENCES Radar Electronics Fundamentals; Navyships 900,016 (T. 0. 161-195); Bureau of Ships, Navy Department: pages 96-98 relied on. 

